Apple - What software alternatives support streaming via AirPlay?

To date, there are no 3rd party software implementations of AirPlay. The spec itself is fully patented (though most of these patents relate to the hardware aspects of the system) and is essentially a proprietary standard unique to Apple (though several hardware vendors have announced hardware that will use the standard - XtremeMac, Denon). The current analysis of the protocol itself seems to indicate that it is a further evolution of the previous AirTunes protocol (which itself was built on top of Bonjour), so you are likely to see software implementations appear after developers have had more time to experiment with the protocol.

Take a look at Airfoil.

AirPlay is effectively a rebranding of AirTunes and adds video/photo streaming. The AirTunes protocol, called RAOP (really just RTSP w/ encryption) remains mostly unchanged. Video streaming is a different animal. My understanding is that its basically push DAAP.

3rd-Party Software
Audio-Only
Airfoil
audio|acacia

Video+Audio
AirServer

For the sake of brevity and time, this list is non-exhaustive and I've only listed commercial software. I've personally noticed that some of the software products touting AirPlay/AirTunes support don't follow the protocols exactly and results vary. I guess you get what you pay for.